1. Field of the Invention
The present invention relates generally to a vehicle speed control system for a motor vehicle having an automatic transmission control system and more particularly to a vehicle speed control system for automatically maintaining the vehicle at a memorized vehicle speed.
2. Description of the Prior Art
In the conventional vehicle speed control system, a throttle valve is controlled so as to maintain the vehicle at a memorized vehicle speed set by a driver. In the conventional automatic transmission, the gear ratio is set to the intended gear ratio by comparing vehicle conditions, such as the vehicle speed, the opening of throttle valve etc., with a shift diagram. Therefore, if the change of opening of the throttle valve is larger than the opening of the throttle valve as controlled by the operation of the vehicle speed control system, the gear ratio in the automatic transmission control system may automatically change in accordance with the shift diagram due to detection of the movement of the throttle valve by the operation of the automatic transmission control system. This change of gear ratio differs from a change of gear ratio in accordance with acceleration or deceleration intended by the driver and occurs at a time unexpected by the driver. Therefore, this change of gear ratio is felt as a shock larger than in a normal gear ratio change intended by the driver More particularly, when driving on a road having many undulations, the change of gear ratio occurs frequently, thereby creating a bad driving sensation.
Furthermore, when the vehicle under speed controlled conditions is climbing an upward slope, the vehicle speed control system causes a hunting operation and cannot maintain the vehicle speed at the memorized vehicle speed whereby the following undesirable operations occur in series:
(1) the vehicle speed is decelerated due to the upward slope of the road,
(2) the throttle valve is controlled in the opening direction by the vehicle speed control system so as to prevent the vehicle from deceleration,
(3) the gear ratio is downshifted due the detection of this change of throttle valve by the automatic transmission control system,
(4) the driving force is increased and as a result, the vehicle speed is accelerated,
(5) the opening of the throttle valve is decreased by vehicle speed control system so as to prevent the vehicle from acceleration,
(6) the gear ratio is upshifted due to detection of this change of opening of the throttle valve by the automatic transmission control system,
(7) the driving force is decreased and as a result, vehicle speed is decreased, and
(8) the opening of the throttle valve is increased by vehicle speed control system so as to prevent the vehicle from deceleration.